The invention relates to a method of oxidizing gaseous substances.
Most known oxidation catalysts for gases such as CO, lower alcohols aldehydes and ketones function very poorly, if at all, in the presence of liquid water and water vapor. Furthermore, these oxidation catalysts function very poorly, if at all, at ambient temperatures.
There is a need for a method of oxidizing these gases wherein the catalyst used will function satisfactorily in the presence of liquid water and water vapor and which function satisfactorily at ambient temperatures.
There is a particular need for a method of oxidizing CO which will function well as ambient temperatures. Carbon monoxide (CO) is a colorless, odorless gas that is extremely toxic. It has an affinity for blood oxyhemoglobin that is over 200 times greater than that of oxygen. The result is that blood will have a reduced capacity for carrying oxygen if breathing air contains CO. The American Council of Government Industrial Hygienists (ACGIH) recommends a threshold limit value time weighted average (TLV-TWA) for CO of 50 ppm for a typical 40 hour work week. Because of a wide variation of individual susceptibility, some people may experience discomfort at levels below the TLV-TWA recommended limit. The acceptable level of CO should be even lower in the domestic environment where, for example, infants and children spend nearly their full day. Thus, there is a need for a process of oxidizing carbon monoxide which works at a low enough temperature and pressure for use in the home or workplace.
There is also a need for a method of oxidizing CO in controlled environmental chambers, such as submarines and undersea diving bells and welding chambers. In at least some of these environments, the humidity levels can be high and the desire for low power consumption is great.
There is a further need for a method of oxidizing CO for an efficient CO removal/filtering device for gas masks used, for example, by firefighters. If a suitable method existed, the need to carry bottled air for breathing could in many instances be eliminated.
There is also a need for low temperature oxidization of other pollutant gases such as lower alcohols, aldehydes and ketones. Such pollutants can arise in the normal household. For instance, formaldehyde is often liberated from synthetic carpets, particle board and cooking. They also arise in many industrial processes.
One well known and commonly used method for carbon monoxide oxidation uses a catalyst that is a proprietary mixture of oxides of manganese and copper marketed under the trade name HOPCALITE, by Mine Safety Appliance Company, Pittsburgh, U.S.A. The catalyst is deactivated by adsorption of water vapor, and can only be used when the relative humidity level is low. The catalyst is therefore kept hot or dried to prevent water adsorption.
A second known method of CO oxidation uses transition metal catalysts. The carbon monoxide reaction on transition metals has been widely studied for many years. See for example Langmuir, I., J., Am. Chem. Soc., 37, 1162, 1915; Trans. Faraday Soc., 621, 1922; Trans. Faraday Soc., 17, 672, 1922. Practical transition metal catalysts for CO oxidation were used, for example, in the first generation of catalytic converters for automobiles. These catalysts consisted of platinum and/or palladium deposited onto a highly porous high surface area support, such as for example alumina in the form either of 1.5-3 mm pellets or a "wash coat" on the walls of a ceramic or metal monolith.
Although alumina is hydrophilic, (that is, it has a tendency to attract and adsorb water vapor) this is not a problem in catalytic converters. As the converters operate at high temperature (500.degree. C. and higher), water vapor present in the gases passing through the converter does not condense in the pores of the alumina support material. Hence the catalyst performance remains high.
At ambient temperatures and high relative humidities, however, alumina does adsorb water, and the pores can rapidly become filled with condensed water vapor. Therefore, the type of catalyst used in the automobile catalytic converter cannot remain active for long periods of time unless it is heated to prevent water adsorption.
Similar catalysts for use at high temperatures are known from French Patent No. 2,063,216 (VEB Synthesewerk Schwartzheide), U.S. Pat. No. 3,804,756 (Callahan) and U.S. Pat. No. 4,323,542 (Joy III). In the French Patent, the catalysts are metal oxides, platinum or palladium, which can be mounted on a support, and the process is carried out at from 250.degree. C. to 500.degree. C. In Callahan, the catalyst is a metal oxide, which may be supported on aluminum, silica or the like, the gaseous substance to be oxidized is entrained in steam, and the process is carried out at 250.degree.-700.degree. C. In Joy III, the catalyst is a specific three-component catalyst comprising uranium, rhodium, and platinum or palladium on alumina, silica or the like and the process is carried out at temperatures above 200.degree. C.
The third known method of CO oxidation uses a catalyst that is a variant of what is commonly referred to as the Wacker process catalyst. In the commercial Wacker process, an aqueous solution of CuCl.sub.2 containing traces of PdCl.sub.2 is used in a vertical reactor to oxidize ethylene partially to acetaldehyde with yields of about 95%. See Thomas, C. L., Catalytic Processes and Proven Catalysts, Academic Press, New York, 1970.
One variant of the Wacker process catalyst that has been shown to be active for oxidation of CO is the catalyst marketed under the trade name Low Temperature Catalyst (LTC) by Teledyne Water-Pik Inc., Fort Collins, Colo., U.S.A. It consists of CuCl.sub.2 with small amounts of PdCl.sub.2, all supported on porous alumina beads. See Collins, M. F., The Catalysis Society, Ninth North Americal Meeting, Houston, Tex., 1985, Mar. 17-21. While this catalyst has been shown to remain active over a wide humidity range, it suffers from loss of performance at humidity levels below about 20% or above 65%. This is a type of supported liquid phase catalyst; it is believed that a surface film of water exists on the alumina, providing a solution medium for the CuCl.sub.2 /PdCl.sub.2 mixture. Inadequate water renders the catalytic agents inactive by reducing their mobility, while too high a level of moisture results in the pores of the alumina support filling with water, thereby increasing the diffusive resistance for the reactants. This latter case is a similar situation to what would occur with the auto exhaust converter type of alumina catalyst.
A carbon catalyst for oxidizing CO to CO.sub.2 at ambient temperatures has also been reported (Tamura, U.S. Pat. No. 4,652,537). The catalyst has a hydrophobic coating put on it by coating with a monomer which is polymerized in situ.